1. Field of the Invention
The present invention relates to wireless communications. More specifically, the present invention relates to improving performance of small co-located cellular networks (e.g., femtocells) under open-access and closed-access configurations.
2. Discussion of the Related Art
In a co-channel femtocell network, the femtocell reuses the spectrum resources of the co-located macrocell network. While a co-channel deployment brings efficient spectrum usage, co-channel deployment also results in co-channel interference (CCI) between the femtocell and the macrocell. The recent technical proposals, “3rd Generation Partnership Project; Technical Specification Group Radio Access Networks; 3G Home NodeB Study Item Technical Report (Release 8),” 3GPP, 3GPP TR 25.820, March 2008, and “Interference management in UMTS femtocells,” FemtoForum, White Paper, December 2008, published online and made available at http://www.femtoforum.org/femto/Files/File/Interference Management in UMTS Femtocells.pdf, provide detailed discussions of different CCI scenarios among femtocell base stations (fBSs), macrocell base station (mBS), and the mobile stations (MSs). Choice of access control at a femtocell is believed to significantly impact the interference that is experienced under different macrocell and femtocell scenarios.
Co-channel femtocell networks may be “open-access” or “closed-access.” In an open-access femtocell, any macrocell MS (mMS) is allowed to join the femtocell. An open-access femtocell often allows a large enough number of users to degrade the average bandwidth available per user. Alternatively, in a closed-access femtocell, e.g., a closed subscriber group (CSG) femtocell, only an authorized group of mMSs may join the femtocell. In a closed-access femtocell, significant interference may result between the femtocell and a close-by co-channel mMS that is not admitted to the femtocell. Simulation results comparing performance between open-access and closed-access femtocells have been reported in (a) “WiMAX femtocells: a perspective on network architecture, capacity, and coverage” (“Yeh”), S. P. Yeh, S. Talwar, S. C. Lee, and H. Kim, IEEE Commun. Mag., vol. 46, no. 10, pp. 58-65, October 2008; (b) “Access methods to WiMAX femtocells: A downlink system-level case study,” D. L. Perez, A. Valcarce, G. D. L. Roche, E. Liu, and J. Zhang; in Proc. IEEE Int. Conf. Commun. Syst. (ICCS), Guangzhou, China, November 2008, pp. 1657-1662; and (c) “Performance of macro- and co-channel femtocells in a hierarchical cell structure,” H. Claussen, in Proc. IEEE Int. Symp. Personal, Indoor, Mobile Radio Commun. (PIMRC), Athens, Greece, September 2007, pp. 1-5. These reports show that open-access mode yields better overall system throughput and coverage. Yeh, in particular, also shows that CSG results in larger areal capacity gains in general. In Yeh's study, areal capacity gain is defined as the ratio of system capacity with one or more femtocells to the corresponding system capacity without a femtocell. Compared to the CSG mode, open-access operations have issues, such as privacy and burden on the backhaul of a femtocell.
In this following description, the term “femtocell” is used to represent a local cellular network that is co-located with a larger cellular network (e.g., a macrocell), the term “femtocell” or “macrocell” are merely illustrative, the principles disclosed herein are equally applicable to any pairing of a local cellular network that is co-located within the service area of a larger cellular network.